This invention relates to a rotation-reciprocation converting motor incorporating a mechanism which converts the rotating operation of an electric motor to a reciprocating operation.
An electric motor of this type has been disclosed, for instance, by Unexamined Japanese Patent Publication 7-27023. FIG. 10 shows the internal structure of a stepper-motor-driven exhaust gas re-circulation control valve which is a motor-driven control device employing a stepper motor. In FIG. 10, a housing 1 has an input port 2 communicated with the exhaust system of an engine, an output port 3 communicated with the suction system of the engine, circulating passageways 4a and 4b, and a water-cooling type passageway 14. A valve seat 6 is press-fitted in the circulating passageway 4a, and fixed with a roll pin 13. Reference numeral 9 designates a bushing serving as a bearing. Reference numeral 8 designates a holder to prevent the entrance of deposits into the bushing 9. The holder 8 is held between the housing 1 and the bushing 9 in such a manner that it is coaxial with the valve seat 6. Reference numeral 5 denotes a valve which is so arranged as to abut against the valve seat. The valve 5 is secured to one end of a valve shaft 7 by caulking. The valve shaft 7 penetrates the bushing 9. A spring holder 10 and a spring washer are secured to the other end of the valve shaft 7 by caulking. Reference numeral 12 designates a spring. The spring 12 is held between the spring holder 10 and the housing 1 to close the valve 5.
Further in FIG. 10, reference numeral 20 designates a stepper motor body. The stepper motor body 20 is coupled to the housing 1 with screws 46 in such a manner that the former 20 is coaxial with the latter 1. Reference numeral 22 designates a bobbin on which a coil 23 is wound. The coil 23 is surrounded by a yoke 24 and a yoke 25. Reference numeral 29 designates a terminal which is electrically connected to the coil 23. The terminal 29 and a motor housing 21 form a connector section. Reference numeral 27 denotes a plate which magnetically shields the two coil sections from each other. Reference numeral 26 designates a plate which prevents resin from flowing into the coil sections when the motor housing 21 is formed by molding. Reference numeral 31 designates a magnet. Reference numeral 32 denotes a rotor which holds the magnet 31, and has a threaded portion 32a in the inner cylindrical surface which is engaged with a threaded portion of a motor shaft 33, and an axial direction stopper 32b for the motor shaft 33. Reference numeral 30 designates bearings set at both ends of the rotor 32. Reference numeral 28 designates a leaf spring for pressing the bearings sidewards. Reference numeral 33 designates the aforementioned motor shaft which has the threaded portion 33a. The threaded portion 33a is engaged with the threaded portion 32a, to convert the rotating motion of the rotor 32 into a linear motion. Reference numeral 34 designates a stopper pin press-fitted in the motor shaft. Reference numeral 41 denotes a bushing which serves as a bearing and is prevented from rotation by means of a hole D-shaped in section. Reference numeral 40 designates a motor holder arranged between the motor housing 21 and the housing 1 in such a manner that it is coaxial with the motor housing 21 and supports the bearing 30 and the motor bushing 41.
The conventional motor designed as described above operates as follows:
In a valve opening operation, a control unit (not shown) applies a pulse voltage to the terminal 29. In response to the voltage, the rotor 31 including the magnet 31 performs a valve opening rotation stepwise. In this operation, the number of signal transmission pulses is equal to the number of steps; that is, an open loop control is carried out accurately. This stepwise rotation is transmitted to the motor shaft 33 through the threaded portion 32a of the rotor 32 and the threaded portion of the motor shaft 33. The motor shaft is regulated in movement in the direction of rotation by a so-called "D-portion" semi-circular in section and the D hole of the bushing 41. Therefore, the rotating motion of the rotor 32 is converted into a linear motion, so that the motor shaft is moved in a valve opening direction (downwardly in FIG. 10).
The rotation-reciprocation converting motor is a stepper motor based on the open loop control. Before the controlling of the motor shaft, it is necessary to perform an initializing operation so that the zero position of the motor is obtained, and with the zero position as a reference, the initial phase of the control unit is made coincident with the initial phase of the motor. That is, the zero position should be determined when the motor shaft is fully contracted or expanded. More specifically, the position is determined as the zero position where the stopper pin 34 abuts against the stopper portion when the motor shaft is fully contracted. Accordingly, in the initializing operation, valve closing signals more than all the number of steps are applied to the motor; that is, by stopping the driving of the motor at the phase which is the same as the phase that the motor shaft is mechanically fully contracted, the control unit is made in phase with the motor.
In the initializing operation, the stopper pin 34 secured to the motor shaft 33, and the rotation regulating D portion 33b of the motor shaft 33 must be in phase in order to perform the initializing operation accurately and to reduce the operating sound in the initializing operation. Further the yoke 24 and the yoke 25 of the motor, the motor bushing 41, the magnet 32a, the threaded portion 31a of the rotor 32, and the stopper portion 32 b of the rotor 32 must also be in phase with one another when positioned in the initializing operation. Furthermore, after the initializing operation, in order to make the amount of protrusion of the shaft 33 in the direction of axis constant, both the phase of the threaded portion 33a of the motor shaft 33, and the phase of the rotation preventing D portion 33b must be made constant.
As conducive to a full understanding of the invention, it is essential to correctly understand the term "phase" as used herein. The term "phase" will be described with reference to FIG. 11 which indicates phase relations between the threaded portion 33a and the rotation regulating portion 33 of the motor shaft 33. In FIG. 11, reference numeral 60 designates a portion where the operation of cutting the threads is started (hereinafter referred to as "a thread start portion" when applicable); 61, a subsidiary line indicating the angular position of the thread start portion 60; and 62, a subsidiary line indicating the central axis of the rotation regulating portion 33b.
In FIG. 11, in order to set the length of the threaded portion 33a to A, the thread start portion 60 is located at a distance A from the right end of the motor shaft 33. In this connection, it should be noted that, the thread start portion 60 may be located at the distance A; however, the threaded start portion 60 cannot be at any position on the cylindrical surface of the motor shaft 33. That is, if it is assumed that the phase is 0.degree. in the case where the thread starts at the position of the subsidiary line 61 (the subsidiary lines 62 and 61 coincide with each other), then in the case where the cutting of the thread is started from an angular position .alpha. with respect to the subsidiary line 62 (from the position below the latter 62 in FIG. 11), the phase is .alpha..degree..
As the rotor 32 is turned around the motor shaft 33, the latter 33 is moved horizontally (right and left) in FIG. 11. In this case, the position of the motor shaft 33 with respect to the angle of rotation of the rotor differs as much as one pitch in maximum depending on the phase of the thread start position. Hence, in order to accurately control the position of the shaft 33 according to the angle of rotation of the rotor 32, the above-described phase should not fluctuate from one product to the next. Although the relation between the threaded portion 33a and the rotation regulating portion 33b has been described, the same thing can be said about other parts.
With the motor thus constructed, in order to accurately achieve the initializing operation, the stopper pin 34, the rotation regulating portion, namely, the D portion 33b, and threaded portion 33a should be coincident in relative positional relation, i.e., in phase with one another. The accurate coincidence of those parts depends on the accuracy of the stopper pin, the accuracy in the press-fitting of the stopper pin into the motor shaft, and the accuracy in positional relation between the threaded portion and the rotation regulating portion. Hence, those parts cannot be adjusted without a number of manufacturing steps. This is not economical. Furthermore, since the stopper pin is press-fitted in the shaft, the former may come off the later. In addition, the stopper pin is; in general, circular in section. Hence, the stopper pin is smaller in its area of abutment against the stopper portion 32b, and therefore the stopper portion 31b is worn out during use, which may result in a small phase shift.